1. Field of the Invention
The present invention relates to a camera module, and more particularly, to a camera module, which comprises a packaged image sensor, an IR filter, a lens and a holder for holding these components and has a reduced overall size and height.
2. Description of the Prior Art
Image sensors are semiconductor devices having the function of photographing images of human beings or objects. The market of these image sensors has been rapidly expanded as they have been loaded into portable phones as well as common digital cameras or camcorders. Such an image sensor is configured in the form of a camera module and mounted in the aforementioned apparatuses. A camera module comprises a lens, a holder, an infrared (IR) filter, an image sensor, and a printed circuit board. An image is formed by the lens of the camera module, the image formed by the lens is concentrated on the image sensor through the IR filter, and an optical signal of the image is converted into an electrical signal by the image sensor so as to photograph the image.
Among these components, the image sensor for converting an optical signal into an electrical signal is directly mounted as a bare chip on the camera module, or mounted on the camera module after an image sensor chip is packaged.
Among several methods of directly mounting a bare chip of an image sensor on a camera module, a COB (Chip-On board) method which currently occupies 90% or more has problems such as low productivity caused by a unit level packaging scheme, a high defective rate caused by introduction of dust particles during fabrication processes, high investment and maintenance costs of equipment including a clean room having a high degree of cleanness, and limitations on miniaturization. That is, all color filters and micro-lenses are very vulnerable to introduction of dust particles or penetration of moisture because they are fabricated through a photolithographic process after being coated with photoresist. Therefore, according to the COB method, the mounting of the image sensor chip, a wiring operation, the installation of the IR filter, lens and holder, and the like should be carried out in a clean room in which a high degree of cleanness is maintained.
On the contrary, if an image sensor that has been packaged in advance is used, it is possible to solve the aforementioned problems caused when the bare chip is used.
FIG. 1 shows a schematic sectional view of a ceramic leadless chip carrier (CLCC) most frequently used as an image sensor package. In a conventional image sensor package 20 shown in the figure, an image sensor chip 22 is mounted on a ceramic substrate 24 by using epoxy or the like such that the surface thereof faces upward, and the image sensor chip is then covered with a glass cover or glass substrate 21. In order to connect the image sensor chip 22 to the ceramic substrate 24, wires 26 connected to the image sensor chip 22 are connected to connection terminals 27 formed on a floor of the ceramic substrate 24, and the image sensor package 20 is connected to a circuit board by the connection terminals 27.
Another package method is to apply a chip scale package scheme (CSP) to an image sensor chip. This method allows an image sensor chip to be packaged at a wafer level contrary to the chip-on board (COB) method in which an image sensor chip as a bare chip is mounted on a camera module, thereby preventing dust or moisture from penetrating into an image sensing area.
An image sensor package 30 shown in FIG. 2 has been proposed by Schellcase Inc. Specifically, an image sensor chip 32 of which a bottom surface is polished to a thickness of about 100 micrometers is first prepared, an adhesive such as epoxy is coated to form an adhesive layer 34 on a top surface of the image sensor chip with a circuit formed thereon, a glass substrate 31 is then attached to the adhesive layer, an adhesive such as epoxy is then coated to form an adhesive layer 33 on the polished bottom surface, and a glass wafer 35 is then attached to the formed adhesive layer. Then, a dicing blade having a slightly gentle tip angle is used to remove a region between the image sensor chip 32 and the adhesive layer 34, thereby exposing input/output pads of the circuit formed on the top surface of the image sensor chip 32. Further, lateral sides of the image sensor chip 32, adhesive layer 33, and glass wafer 35 are formed to be inclined at a certain angle by using equipment such as a semiconductor wafer cutter (dicing saw). Next, metal wires 36 are formed to extend from the input/output pads of the exposed image sensor chip 32 via the inclined lateral side surfaces to a bottom surface of the glass wafer 35. At this time, the metal wires 36 are formed by forming a metal film from the input/output pads of the exposed image sensor chip 32 via the inclined lateral side surfaces to the bottom surface of the glass wafer 35 and by etching the metal film to form a desired pattern. Finally, connection terminals 37 such as solder balls are formed at ends of the metal wires 36 formed on the bottom surface of the glass wafer 35. The connection terminals 37 will be connected to external terminals or a printed circuit board (PCB). Such an image sensor package available from Shellcase Inc. can be completed to conform to the size of an actual image sensor chip.
As another example of CSP, an image sensor package proposed by the present applicant shown in FIGS. 3A and 3B.
The image sensor package 40 of FIG. 3A comprises a glass substrate 41, metal wires 44 formed on the glass substrate 41, an insulating film 45 for protecting the metal wires 44, an image sensor chip 42 electrically connected to the glass substrate 41 by flipchip solder joints 43, and connection terminals 47 such as solder balls formed outside the image sensor chip 42 and connected to a printed circuit board. Meanwhile, a dust-seal layer 46 is formed between the glass substrate 41 and the image sensor chip 42 to prevent foreign substances from being introduced into a space defined between the glass substrate 41 and the image sensor chip 42.
An image sensor package 50 for a camera module shown in FIG. 3B comprises a glass substrate 51, metal wires 54 formed on the glass substrate 51, an insulating film 55 for protecting the metal wires 54, an image sensor chip 52 electrically connected to the glass substrate 51 by flipchip solder joints 53, and passive elements 58 and connection terminals 57 mounted on the metal wires 54 outside the image sensor chip 52. Although the image sensor package 50 shown in FIG. 3B has a structure nearly similar to that of the image sensor package 40 shown in FIG. 3A, it has a structure in which the passive elements 58, such as decoupling capacitors, required to construct a camera module can be mounted together on the glass substrate and the connection terminals 57 for connection with a printed circuit board are provided on one surface of the glass substrate. Therefore, in case of such an image sensor package, it is basically possible to eliminate a printed circuit board in fabricating a camera module.
In addition to the aforementioned package methods, a further package method is to apply a chip on flexible PCB (COF) to an image sensor chip.
An example of an image sensor package manufactured according to the COF package method is shown in FIG. 4. Referring to FIG. 4, an image sensor package 60 includes an image sensor chip 62, a flexible PCB 64, which is formed with printed circuit patterns 64p on a surface of the flexible PCB 64, formed with a through-hole 64h for an image sensing portion of the image sensor chip 62 to be seen, and connected to the image sensor chip 62, and a glass substrate 61 attached to the other surface of the flexible PCB 64 so as to cover the through-hole 64h of the flexible PCB 64.
At this time, portions of the printed circuit patterns 64p around the through-hole 64h of the flexible PCB 64 are electrically connected to pads formed on edges of the image sensor chip 62. That is, using an anisotropic conductive film (ACF) or anisotropic conductive paste (ACP), connecting portions 65 between the printed circuit patterns 64p of the flexible PCB 64 and the pads of the image sensor chip 62 electrically connect the printed circuit patterns 64p of the flexible PCB 64 and the pads of the image sensor chip 62 to each other.
Generally, in the image sensor package 60 of a COF type, when the ACF or ACP is pressed after forming gold bumps on input and output pads of the image sensor chip 62, polymer balls in the ACF or ACP which have conductive surfaces are compressed, so that the electrical connection is achieved. The detailed descriptions thereof will be omitted since the configurations thereof are well known to those skilled in the art.
At this time, the edges of the image sensor chip 62 and the flexible PCB 64 may be bonded with epoxy 66 and the like.
In the mean time, an IR filter or the like may be used as the glass substrate 61, which serves to protect the image sensor chip 62. At this time, the glass substrate 61 and the flexible PCB 64 may be bonded to each other with adhesive or double-sided adhesive tape to be airtight. Next, the structure of a conventional camera module will be described in greater detail with reference to the drawings. First, a conventional camera module having the image sensor package 40 of FIG. 3A among the aforementioned image sensor packages will be described.
As shown in the figure, the conventional camera module comprises the aforementioned image sensor package 40, a printed circuit board 10 on which the image sensor package 40 is mounted, a holder 12 having a lower end fixed to the printed circuit board 10, an IR filter 14 installed at the center of the holder 12, and a lens unit 18 coupled to a lens mounting portion 12c formed at the center of an upper end of the holder 12.
In order to mount the image sensor package 40 on the printed circuit board 10, connection terminals 47 of the image sensor package 40 are soldered to printed circuit patterns (not shown) on the printed circuit board 10. Further, in order to attach the holder 12 to the printed circuit board 10 and attach the IR filter 14 to the holder 12, the IR filter 14 is first bonded to the holder 12 using epoxy 15. Thereafter, epoxy 11 in the form of paste is applied to an edge of the printed circuit board 10, the holder 12 with the IR filter 14 bonded thereto is attached to the epoxy 11 in the form of paste applied to the printed circuit board 10, and the epoxy in the form of paste is then cured at a predetermined temperature to firmly bond the printed circuit board 10 and the holder 12 to each other. When the holder 12 is completely attached to the printed circuit board 10, the lens unit 18 is coupled to the lens mounting portion 12c formed at the center of the upper end of the holder 12.
The lens unit 18 is constructed such that at least one lens 18a is fixedly installed within a hollow cylindrical case, and male threads formed on an outer surface of the case are engaged with female threads formed on an inner surface of the lens mounting portion 12c of the holder 12c. When the lens unit 18 is coupled to the lens mounting portion 12c, the distance between the lens unit 18 and the image sensor package 40, i.e., a focal distance, is adjusted.
In the camera module having such a structure, the thickness of a sidewall of the holder 12 is about 350 to 450 μm, and a tolerance of about 150 μm is required for assembling between the inner wall of the holder 12 and an edge of the glass substrate 41 of the package 40. In the camera module assembled in this way, since the size of the holder 12 is standardized to 5 mm×5 mm, 6 mm×6 mm and the like for application of the camera module to an electronic apparatus, a standardized holder always having a size larger than the size of the image sensor package 40 to be used is employed, resulting in increase in the overall size of the camera module.
Meanwhile, since printed circuit patterns (not shown) are formed on the top surface of the printed circuit board 10, the top surface is not generally even. For this reason, when the holder 12 is attached to the printed circuit board 10, a double-sided adhesive tape cannot be used but epoxy in the form of paste is applied and cured. Such epoxy paste generates vapor or particles during a curing process thereof. At this time, since the holder 12 is hermetically sealed, the vapor or particles do not escape the camera module but adhere to the image sensor package 40 or the IR filter 14. The vapor or particles adhering to the image sensor package 40 or the IR filter 14 may contaminate the image sensor package and IR filter, leading to distortion of a photographed image.
When epoxy in the form of paste is applied and cured on the uneven printed circuit board 10 in order to bond the holder 12 to the printed circuit board as described above, the thickness of the epoxy 11 interposed between the printed circuit board 10 and the printed circuit board 12 varies depending on camera modules to be manufactured. That is, after the epoxy in the form of paste is applied and cured, it is not easy to obtain cured epoxy 11 having an initially intended thickness.
For this reason, the lens unit 18 is manufactured separately and then threadly engaged with the lens mounting portion 12c formed in the holder 12 while adjusting the focal distance between the lens unit 18 and the image sensor package 40 after the printed circuit board 10 and the holder 12 are completely assembled together. That is, even though the same lens unit 18 and image sensor unit 40 are used, the focal distance between the holder 12 with the lens unit 18 mounted thereon and the printed circuit board 10 with the image sensor package 40 mounted thereon varies depending on the thickness of the cured epoxy 11. Accordingly, the focal distance between the lens unit 18 and the image sensor package 40 should be adjusted depending on the thickness of the epoxy 11.
Since the process of adjusting the focal distance between the lens unit 18 and the image sensor package 40 should be individually performed for every camera module to be manufactured, it is a process that requires a great deal of time and is very difficult to be automated among all the processes of manufacturing a camera module. Since the process of adjusting the focal distance requires a great deal of time and manpower as described above, the process increases manufacturing costs of camera modules.
In particular, since the thickness of the cured epoxy 11 may not be uniform even in one camera module, the holder 12 may be obliquely bonded to the printed circuit board 10. This causes misalignment between a lens and an image sensor, leading to a defective product. In particular, in a high pixel camera module on the degree of mega pixels, distortion of a final image is apt to occur due to the misalignment.
Meanwhile, since the image sensor packages 20 and 30 shown in FIGS. 1 and 2 are also formed with the connection terminals 27 and 37 on their bottom surfaces, respectively, similarly to the image sensor package 40 shown in FIG. 3A and assembled into a camera module in the same manner, they have the same problems. Moreover, since the image sensor package 50 shown in FIG. 3B is used without a printed circuit board, the image sensor package 50 is assembled into a camera module in the same manner as described above after a reinforcing plate instead of a printed circuit board is installed below the image sensor package 50 and a holder is bonded to the top of the reinforcing plate. This image sensor package also has the same problems.